This invention relates to an electrode for medical applications, particularly an implantable defibrillation electrode.
Electrodes implanted in the body for electrical defibrillation are well known. More specifically, electrodes implanted on or about the heart have been used to reverse (i.e., defibrillate or cardiovert) certain life-threatening cardiac arrhythmias by applying electrical energy to the heart via these electrodes to return the heart to normal sinus rhythm. The amount of energy delivered to the heart during defibrillation (or cardioversion) depends on the placement of the electrodes on the heart and the ability of the electrode to distribute the energy applied as input to the electrodes through the heart. This energy is called the defibrillation or cardioversion energy.
In defibrillation, two opposing electrodes are typically implanted on or about the heart to create an electric field across and through the heart. This field eliminates the sporadic electrical impulses present throughout the heart that characterize fibrillation. Factors that determine the shape and strength of the field in the heart, and the efficiency with which the field is generated, are the shape of the electrodes, the amount of reactive surface area of the electrodes, the placements of the electrodes and the selection of the electrode materials. All of these factors have the effect of reducing the electrode interface impedance allowing the transfer of more energy to the heart tissue between the electrodes. All implantable devices have a finite amount of energy available making it imperative that discharge is delivered as efficiently as possible. This is particularly important when dealing with high energy (10-15 Joule) defibrillation shocks.
There have been attempts to develop desirable field shapes and discharge energies by the use of two electrodes in patch form placed in the region of the heart. See, for example, U.S. Pat. Nos. to Heilman et al. (4,030,509 and 4,291,707) and European Patent Application No. 0,280,564 to Ideker. Although such two patch electrode arrangements are effective, the electrodes may become so large or be placed so closely together that there is a danger of an electrical short-circuit or low impedance shunt developing between the two electrodes. Once that short develops, no effective electric field is generated and defibrillation cannot occur.
U.S. Pat. No. to Enger (4,011,861) discloses an implantable electrode having conductive porous layers formed into particular electrode shapes. The Enger device is described for use as a heart sensor or stimulator and comprises a disc of conductive porous material attached to a layer of inner non-absorbable conductive porous material. This porous structure is said to allow intermeshing with the tissue and permit ingress of blood vessels without the production of a fibrous tissue in-growth interface. Moreover, it is stated that body electrolytes can fill the interstices of the porous layer to contact the electrically conductive source of the electrode.
U.S. Pat. No. to Cannon (3,981,309) discloses a stimulating pacing electrode of a cylindrical shape formed of a conductive porous material such as platinum dust. The patent contends that the resulting multiplicity of interstices provides a large surface area to reduce polarization losses and to facilitate small electrode dimensions for increasing the stimulation current density. Both the Enger and Cannon electrode designs are for use in pacemaker systems, not in higher energy defibrillation systems.
Other implantable electrodes are disclosed in U.S. Pat. Nos. to DeHaan et al. (4,649,937) and Hirshorn et al. (4,407,302). The DeHann et al. patent discloses an implantable electrode tip member having a plurality of grooves etched in the bullet shaped distal end. The patent purports that an enlarged surface area is created on a relatively small tip to ensure sufficient current flow into the organ tissue. The Hirshorn et al. patent discloses a cardiac pacemaker electrode tip structure having its external surface roughened by abrading with a jet of glass beads projected under pressure. This etching is stated to increase the microsurface area of the tip resulting in a decrease of the sensing impedance and an increase in the pacing impedance of the electrode. As a result, less current drain on the pacemaker power source is said to occur increase the life span of the pacemaker. While both patents describe methods to increase surface area, neither discusses the use of the increased surface area for direct blood/tissue contact in a defibrillation environment.
Further porous structures of implantable electrodes are discussed in the U.S. Pat. Nos. to Hirshorn et al. (4,408,604) and MacGregor (4,281,669). The MacGregor patent discloses a pacemaker electrode having a distal tip end formed of porous material to permit the formation of a smooth adherent tissue coating without the formation of blood clots. This electrode is said to achieve very high current density at several points on the surface while exerting a low energy drain from the battery. The Hirshorn et al. patent discloses a similar porous pacemaker electrode tip member which enables tissue to grow into the apertures to facilitate attachment to a body organ. In addition, it is stated in the patent that this structure provides a large microsurface area resulting in a low sensing impedance to improve sensing capability. Neither the Hirshorn et al. device nor the MacGregor device uses the porous structure to increase the surface contact area or to allow body fluids to enter the electrode. Also, neither of these electrodes are described as for use in implantable defibrillation systems.
U.S. Pat. Nos. to Mund et al. (4,603,704) and Parsonnet et al. (3,476,116) disclose pacemaker electrodes having means to provide direct blood contact with the electrical conductor of the electrode. The Mund et al. device comprises an elongated electrically conductive carrier material surrounded by porous layers. This porous system is said to provide a high double layer capacitance to keep the energy consumption low. The Parsonnet et al. device comprises a small container completely enclosed except for a small hole. Platinum foil is placed inside the container and connected to a pulse generator. The container is filled with saline solution or blood and placed in the body with the hole facing the heart surface. The patent states that all of the current on the foil must pass through the hole at a very high density with a substantial reduction of polarization effects. Both of these designs require intricate constructions and also are not applicable to defibrillation techniques.
Other attempts to lower pacing threshold energies through the use of drug elution are described in the U.S. Pat. Nos. to Stokes (4,506,680 and 4,577,642) and White (4,360,031).